Hot-rolled steel sheet

ABSTRACT

A hot-rolled steel sheet includes a predetermined chemical composition, and a structure which includes, by area ratio, ferrite and bainite in a range of 75% to 95% in total, and martensite in a range of 5% to 20%, in which in the structure, in a case where a boundary having an orientation difference of equal to or greater than 15° is set as a grain boundary, and an area which is surrounded by the grain boundary, and has an equivalent circle diameter of equal to or greater than 0.3 μm is defined as a grain, the ratio of the grains having an intragranular orientation difference in a range of 5° to 14° is, by area ratio, in a range of 10% to 60%.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a hot-rolled steel sheet excellent inworkability and particularly relates to a hot-rolled steel sheet havinga composite structure and excellent in stretch flangeability.

RELATED ART

In recent years, in response to the demand for reduction in weight ofvarious members for the purpose of improving fuel economy of vehicles, areduction in thickness was accomplished by increasing the strength of asteel sheet such as an iron alloy used for the members, and applicationof light metals such as an Al alloy to the various members have beenproceeded. However, as compared with heavy metals such as steel, thelight metals such as an Al alloy have an advantage of high specificstrength, but are extremely expensive. For this reason, the applicationof the light metal such as an Al alloy is limited to specialapplications. Accordingly, in order to apply the reduction in the weightof the various members to a cheaper and wider range, it is required toreduce the thickness by increasing the strength of the steel sheet.

When the steel sheet is strengthened, the material properties such asformability (workability) are generally deteriorated. Thus, in thedeveloping of the high-strength steel sheet, it is an important problemto achieve the high strength of the steel sheet without deterioratingthe material properties. Particularly, stretch-flange formability,burring workability, ductility, fatigue durability, impact resistance,corrosion resistance, and the like are required for the steel sheet usedas vehicle members such as an inner plate member, a structural member,and a suspension member, depending on the application, and it isimportant to realize both of material properties and strength.

For example, among the vehicle members, the steel sheets used for thestructural member, the suspension member, and the like, which accountfor about 20% of the vehicle body weight are press-formed mainly basedon stretch flange processing and burring processing after performingblanking and drilling by shearing or punching. For this reason,excellent stretch flangeability is required for such steel sheets.

With respect to the above-described problem, for example, PatentDocument 1 discloses a hot-rolled steel sheet in which the fraction andthe size of the martensite, the number density, and the average gapbetween martensite is specified, and is excellent in elongation and holeexpansibility. Patent Document 2 discloses a hot-rolled steel sheet inwhich average particle diameters of ferrite and a second phase and acarbon concentration of the second phase are limited, and is excellentin burring workability. Patent Document 3 discloses a hot-rolled steelsheet which is obtained by coiling the steel sheet at a low temperatureafter being kept at a temperature in a range of 750° C. to 600° C. for 2to 15 seconds, and is excellent in workability, surface texture, andplate flatness.

However, in Patent Document 1, since a primary cooling rate should beset to be equal to or higher than 50° C./s after completing the hotrolling, the load applied on an apparatus becomes higher. In addition,in a case of setting the primary cooling rate to be equal to or higherthan 50° C./s, there is a problem in that unevenness in materialproperties is caused by unevenness in the cooling rate.

In addition, as described above, in recent years, the demand for thehigh-strength steel sheet to the automobile members have been required.In a case where the high-strength steel sheet is press-formed by coldworking, cracks likely to occur at an edge of a portion which issubjected to the stretch flange forming during the forming process. Thereason for this is that work hardening occurs only on an edge portiondue to the strain which is introduced to a punched end surface at thetime of blanking. In the related art, as a method of evaluation a testof the stretch flangeability, a hole expansion test has been used.However, in the hole expansion test, breaking occurs without the strainsin the circumferential direction are hardly distributed; however, in theactual process of components, strain distribution is present, and thus agradient of the strain and the stress in the vicinity of the brokenportion affects a breaking limit. Accordingly, regarding thehigh-strength steel sheet, even if the stretch flangeability issufficient in the hole expansion test, in a case of performing coldpressing, the breaking may occur due to the strain distribution.

The techniques disclosed in Patent Documents 1 to 3 disclose that in allof the inventions, the hole expansibility is improved by specifying onlythe structures observed using an optical microscope. However, it is notclear whether or not sufficient stretch flangeability can be securedeven in consideration of the strain distribution.

PRIOR ART DOCUMENT Patent Document

[Patent Document 1] Japanese Unexamined Patent Application, FirstPublication No. 2013-19048

[Patent Document 2] Japanese Unexamined Patent Application, FirstPublication No. 2001-303186

[Patent Document 3] Japanese Unexamined Patent Application, FirstPublication No. 2005-213566

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

The present invention has been made in consideration of theabove-described circumstance.

An object of the present invention is to provide a high-strengthhot-rolled steel sheet which is excellent in the stretch flangeabilityand can be applied to a member which requires high strength and thestrict stretch flangeability. In the present invention, the stretchflangeability means a value evaluated by a product of limit formingheight H (mm) and tensile strength TS (MPa) of the flange obtained as aresult of the test by the saddle type stretch flange test method, whichis an index of the stretch flangeability in consideration of the straindistribution. In addition, the excellent stretch flangeability meansthat the product of the limit forming height H (mm) and the tensilestrength TS (MPa) is equal to or greater than 19500 (mm·MPa). Inaddition, the high strength means that the tensile strength is equal toor greater than 590 MPa. There is no need to particularly set the upperlimit of the strength; however, in the range of the structure defined inthe present invention, it is difficult to secure a strength of greaterthan 1470 MPa.

Means for Solving the Problem

According to the related art, the improvement of the stretchflangeability (hole expansibility) has been performed by inclusioncontrol, homogenization of structure, unification of structure, and/orreduction in hardness difference between structure, as disclosed inPatent Documents 1 to 3. In other words, in the related art, holeexpansibility, workability, or the like have been improved bycontrolling the structure which can be observed using an opticalmicroscope.

In this regard, the present inventors made an intensive study byfocusing an intragranular orientation difference in grains inconsideration that the stretch flangeability under the presence of thestrain distribution cannot be improved even by controlling only thestructure observed using an optical microscope. As a result, it wasfound that it is possible to greatly improve the stretch flangeabilityby controlling the ratio of the grains in which the intragranularorientation difference is in a range of 5° to 14° with respect to theentire grains to be within a certain range.

The present invention is configured on the basis of the above findings,and the gists thereof are as follows.

(1) A hot-rolled steel sheet according to one aspect of the presentinvention includes, as a chemical composition, by mass %, C: 0.04% to0.18%, Si: 0.10% to 1.70%, Mn: 0.50% to 3.00%, Al: 0.010% to 1.00%, B:0% to 0.005%, Cr: 0% to 1.0%, Mo: 0% to 1.0%, Cu: 0% to 2.0%, Ni: 0% to2.0%, Mg: 0% to 0.05%, REM: 0% to 0.05%, Ca: 0% to 0.05%, Zr: 0% to0.05%, P: limited to equal to or less than 0.050%, S: limited to equalto or less than 0.010%, and N: limited to equal to or less than 0.0060%,with the remainder including of Fe and impurities; and a structure whichincludes, by area ratio, a ferrite and a bainite in a range of 75% to95% in total, and a martensite in a range of 5% to 20%, in which in thestructure, in a case where a boundary having an orientation differenceof equal to or greater than 15° is defined as a grain boundary, and anarea which is surrounded by the grain boundary and has an equivalentcircle diameter of equal to or greater than 0.3 μm is defined as agrain, the ratio of the grains having an intragranular orientationdifference in a range of 5° to 14° is, by area ratio, in a range of 10%to 60%.

(2) In the hot-rolled steel sheet described in the above (1), a tensilestrength may be equal to or greater than 590 MPa, and a product of thetensile strength and a limit forming height in a saddle type stretchflange test may be equal to or greater than 19500 mm·MPa.

(3) In the hot-rolled steel sheet described in the above (1) or (2), thechemical composition may contain, by mass %, one or more selected fromthe group consisting of: B: 0.0001% to 0.005%, Cr: 0.01% to 1.0%, Mo:0.01% to 1.0%, Cu: 0.01% to 2.0%, and Ni: 0.01% to 2.0%.

(4) In the hot-rolled steel sheet described in any one of the above (1)to (3), the chemical composition may contain, by mass %, one or moreselected from the group consisting of: Mg: 0.0001% to 0.05%, REM:0.0001% to 0.05%, Ca: 0.0001% to 0.05%, and Zr: 0.0001% to 0.05%.

Effects of the Invention

According to the above-described aspects of the present invention, it ispossible to provide a high-strength hot-rolled steel sheet which hashigh strength, can be applied to a member that requires strict stretchflangeability, and is excellent in stretch flangeability.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an analysis result obtained by EBSD at ¼t portion (a ¼thickness position from the surface in the sheet thickness direction) ofa hot-rolled steel sheet according to the present embodiment.

FIG. 2 is a diagram showing a shape of a saddle-shaped formed productwhich is used in a saddle type stretch flange test method.

EMBODIMENTS OF THE INVENTION

Hereinafter, a hot-rolled steel sheet (hereinafter, referred to as ahot-rolled steel sheet according to the present embodiment in some case)of the embodiment of the present invention will be described in detail.

The hot-rolled steel sheet according to the present embodiment includes,as a chemical composition, by mass %, C: 0.04% to 0.18%, Si: 0.10% to1.70%, Mn: 0.50% to 3.00%, Al: 0.010% to 1.00%, and optionally B: 0.005%or less, Cr: 1.0% or less, Mo: 1.0% or less, Cu: 2.0% or less, Ni: 2.0%or less, Mg: 0.05% or less, REM: 0.05% or less, Ca: 0.05% or less, Zr:0.05% or less, P: limited to equal to or less than 0.050%, S: limited toequal to or less than 0.010%, and N: limited to equal to or less than0.0060%, with the remainder including Fe and impurities.

In addition, in the hot-rolled steel sheet according to the presentembodiment, a structure includes, by area ratio, ferrite and bainite ina range of 75% to 95% in total, and martensite in a range of 5% to 20%.In addition, in the structure, when a boundary having an orientationdifference of equal to or greater than 15° is defined as a grainboundary, and an area which is surrounded by the grain boundary and hasan equivalent circle diameter of equal to or greater than 0.3 μm isdefined as a grain, the ratio of the grains having an intragranularorientation difference in a range of 5° to 14° is, by area ratio, in arange of 10% to 60%.

First, the reason for limiting the chemical composition of thehot-rolled steel sheet according to the present embodiment will bedescribed. The amount (%) of the respective elements is based on mass %.

C: 0.04% to 0.18%

C is an element which contributes to improvement of the strength ofsteel. In order to obtain the aforementioned effect, the lower limit ofthe C content is set to 0.04%. In addition, when the C content is lessthan 0.04%, the ratio of the grains having an intragranular orientationdifference in a range of 5° to 14° is decreased. From this point, thelower limit of the C content is set to 0.04%. The lower limit of the Ccontent is preferably 0.045%, and the lower limit of the C content isfurther preferably 0.05%. On the other hand, when the C content isgreater than 0.18%, the stretch flangeability and the weldability aredeteriorated. Further, the hardenability is excessively enhanced, andthe grains having an intragranular orientation difference of greaterthan 14° are increased, thereby the ratio of grains having anintragranular orientation difference in a range of 5° to 14° isdecreased. Thus, the upper limit of the C content is set to 0.18%. Theupper limit of the C content is preferably 0.17%, and the upper limit ofthe C content is further preferably 0.16%.

Si: 0.10% to 1.70%

Si is an element which contributes to improvement of the strength ofsteel. In addition, Si is an element having a role as a deoxidizingagent of molten steel. In order to obtain the aforementioned effect, thelower limit of the Si content is set to 0.10%. The lower limit of the Sicontent is preferably 0.12%, the lower limit of the Si content isfurther preferably 0.15%. On the other hand, when the Si content isgreater than 1.70%, since Ar3 transformation temperature becomesexcessively high, it is difficult to perform hot rolling in a γ region,processed ferrite is generated, and the ratio of the grains having anintragranular orientation difference in a range of 5° to 14° isdecreased, thereby deteriorating the stretch flangeability. For thisreason, the upper limit of the Si content is set to 1.70%. The upperlimit of the Si content is preferably 1.60%, and the upper limit of theSi content is further preferably 1.50%.

Mn: 0.50% to 3.00%

Mn is an element which contributes to the improvement of the strength ofsteel by the solid solution strengthening and/or improving thehardenability of the steel. In order to obtain the aforementionedeffect, the lower limit of the Mn content is set to 0.50%. The lowerlimit of the Mn content is preferably 0.65%, and the lower limit of theMn content is further preferably 0.70%. On the other hand, when the Mncontent is greater than 3.00%, the ratio of the grains having anintragranular orientation difference in a range of 5° to 14° isdecreased, and thereby the stretch flangeability is deteriorated. Forthis reason, the upper limit of the Mn content is set 3.00%. The upperlimit of the Mn content is preferably 2.6%, and is further preferablythe upper limit of the Mn content is 2.30%.

Al: 0.010% to 1.00%

Al is an effective element as a deoxidizing agent of molten steel. Inorder to obtain such effects, the lower limit of the Al content is setto 0.010%. The lower limit of the Al content is preferably 0.015%, andthe lower limit of the Al content is further preferably 0.020%. On theother hand, the Al content is greater than 1.00%, the weldability andthe toughness are deteriorated. For this reason, the upper limit of theAl content is set to 1.00%. The upper limit of the Al content ispreferably 0.90%, and the upper limit of the Al content is furtherpreferably 0.80%.

P: Equal to or Less than 0.050%

P is an impurity. P causes the toughness, the workability, and theweldability to be deteriorated, and thus the less the content, thebetter. However, in a case where the P content is greater than 0.050%,the stretch flangeability is remarkably deteriorated, and thus the Pcontent is limited to be equal to or less than 0.050%. The P content isfurther preferably equal to or less than 0.040%. Although, there is noneed to particularly specify the lower limit of the P content, excessivereduction of the P content is undesirable from the viewpoint ofmanufacturing cost, and thus the P content may be equal to or greaterthan 0.005%.

S: Equal to or Less than 0.010%

S is an element for forming an A type inclusion which not only causescracks at the time of hot rolling, but also makes the stretchflangeability deteriorated. For this reason, the less the S content, thebetter. However, when the S content is greater than 0.010%, the stretchflangeability is remarkably deteriorated, and thus the upper limit ofthe S content is limited to 0.010%. The S content is further preferablyequal to or less than 0.005%. Although, there is no need to particularlyspecify the lower limit of the S content, excessive reduction of the Scontent is undesirable from the viewpoint of manufacturing cost, andthus the S content may be equal to or greater than 0.001%.

N: Equal to or Less than 0.0060%

N is an element which forms AlN during the cooling after hot rolling,and deteriorates the formability of the steel sheet. Particularly, in acase where the N content is greater than 0.0060%, the stretchflangeability is remarkably deteriorated. For this reason, the upperlimit of the N content is limited to be equal to or less than 0.0060%.The upper limit of the N content is preferably 0.0040%. Although, thereis no need to particularly specify the N content, excessive reduction ofthe N content is undesirable from the viewpoint of manufacturing cost,and thus the lower limit of the N content may be equal to or greaterthan 0.0010%.

The above-described chemical elements are base elements contained in thehot-rolled steel sheet according to the present embodiment, and achemical composition which contains such basic elements, with theremainder including Fe and impurities is a base composition of thehot-rolled steel sheet according to the present embodiment. Theimpurities are elements contaminated in the steel, which are caused fromraw materials such as ore and scrap at the time of industriallymanufacturing an alloy such as As and Sn, or caused by various factorsin the manufacturing process, and are in an allowable range which doesnot adversely affect the properties of the hot-rolled steel sheetaccording to the present embodiment.

However, for the purpose of further improving the strength and thetoughness, the hot-rolled steel sheet further contains, if necessary,one or more of B, Cr, Mo, Cu, Ni, Mg, REM, Ca, and Zr within a rangedescribed below. It is not necessary to contain these elements, and thusthe lower limit of the content is 0%. Among the aforementioned elements,Nb and Ti limit the recrystallization and thus the workability isdeteriorated. For this reason, Nb is preferably less than 0.005%, and Tiis preferably less than 0.015.

B: 0.0001% to 0.0050%

B is an element which improves the hardenability, and contributes tostrengthening of steel. In order to obtain the aforementioned effect,the B content is preferably set to be equal to or greater than 0.0001%.On the other hand, when the B content is greater than 0.0050%, theworkability is deteriorated. In addition, bainite having a largeorientation dispersion is likely to be generated at the time ofquenching, and the ratio of the grains having an intragranularorientation difference in a range of 5° to 14° is decreased. For thisreason, even in a case of containing B, the upper limit of the B contentis preferably 0.0050%.

Cr: 0.01 to 1.0%

Cr is an element which contributes to improvement of the strength ofsteel. In addition, Cr is an element having an effect of limitingcementite. In a case of obtaining such effects, the Cr content ispreferably equal to or greater than 0.01%. On the other hand, when theCr content is greater than 1.0%, the ductility is deteriorated.Accordingly, even in a case of containing Cr, the upper limit of the Crcontent is preferably 1.0%.

Mo: 0.01% to 1.0%

Mo is an element which improves the hardenability and has an effect ofenhancing the strength by forming a carbide. In order to obtain sucheffects, the Mo content is preferably equal to or greater than 0.01%. Onthe other hand, when the Mo content is greater than 1.0%, the ductilityand the weldability are deteriorated. For this reason, the upper limitof the Mo content is set to 1.0% even in a case of containing Mo.

Cu: 0.01% to 2.0%

Cu is an element which enhances the strength of steel sheet and improvescorrosion resistance and the exfoliation properties of the scale. Inorder to obtain such effects, the Cu content is preferably equal to orgreater than 0.01%, and is further preferably equal to or greater than0.04%. On the other hand, when the Cu content is greater than 2.0%, itis concerned that surface flaws occur. For this reason, even in the caseof containing Cu, the upper limit of the Cu content is preferably set to2.0%, and is further preferably set to 1.0%.

Ni: 0.01% to 2.0%

Ni is an element which enhances the strength and improves the toughnessof the steel sheet. In order to obtain such effects, the Ni content ispreferably equal to or greater than 0.01%. On the other hand, when theNi content is greater than 2.0%, the ductility is deteriorated. For thisreason, even in the case of containing Ni, the upper limit of the Nicontent is preferably set to 2.0%.

Ca: 0.0001% to 0.05%

Mg: 0.0001% to 0.05%

Zr: 0.0001% to 0.05%

REM: 0.0001% to 0.05%

All of Ca, Mg, Zr, and REM are elements which improve the toughness bycontrolling the shape of sulfides and oxides. Accordingly, in order toobtain such effects, each of one or more of these elements is preferablyequal to or greater than 0.0001%, and is further preferably equal to orgreater than 0.0005%. However, when the amount of these elements isexcessively high, the stretch flangeability is deteriorated. For thisreason, even in the case of containing these elements, the upper limitof each content is preferably set to 0.05%.

Next, the structure (metallographic structure) of the hot-rolled steelsheet according to the present embodiment will be described.

It is necessary that the hot-rolled steel sheet according to the presentembodiment contain, by area ratio, ferrite and bainite in a range of 75%to 95% in total, and martensite in a range of 5% to 20%, in thestructure observed using an optical microscope. With such a compositestructure, it is possible to improve the strength and the stretchflangeability in good balance. When the total amount of the ferrite andthe bainite is less than 75% by area ratio, the stretch flangeability isdeteriorated. In addition, when the total area ratio of the ferrite andthe bainite is greater than 95%, the strength is deteriorated, theductility is deteriorated, and thereby it is difficult to secure theproperties which are generally required for the vehicle members.Although each of the fraction (the area ratio) of the ferrite and thebainite is not necessarily limited, when the fraction of the ferrite isgreater than 90%, sufficient strength cannot be obtained in some cases,and thus the fraction of the ferrite is preferably equal to less than90%, and is further preferably less than 70%. On the other hand, whenthe fraction of the bainite is greater than 60%, the ductility may bedeteriorated, and thus the fraction of the bainite is preferably lessthan 60%, and is further preferably less than 50%.

In the hot-rolled steel sheet according to the present embodiment, thestructures of the remainders other than the ferrite, bainite, andmartensite are not particularly limited, and for example, it may beresidual austenite, pearlite, or the like. However, when the structuresof the remainder other than the ferrite, bainite, and martensite aregreater than 5% in total, the stretch flangeability and the ductilityare deteriorated. For this reason, the ratio of the structures of theremainders is preferably equal to or less than 5%, further preferablyequal to or less than 3%, and still further preferably 0%, by arearatio.

The structure fraction (the area ratio) can be obtained using thefollowing method. First, a sample collected from the hot-rolled steelsheet is etched using nital. After etching, a structure photographobtained at a ¼ thickness position in a visual field of 300 μm×300 μmusing an optical microscope is subjected to image analysis, and therebythe area ratio of ferrite and pearlite, and the total area ratio bainiteand martensite are obtained. Then, a sample etched by LePera solution,the structure photograph obtained at a ¼ thickness position in thevisual field of 300 μm×300 μm using the optical microscope is subjectedto the image analysis, and thereby the total area ratio of residualaustenite and martensite is calculated.

Further, with a sample obtained by grinding the surface to a depth of ¼thickness from the normal direction to the rolled surface, the volumefraction of the residual austenite is obtained through X-ray diffractionmeasurement. The volume fraction of the residual austenite is equivalentto the area ratio, and thus is set as the area ratio of the residualaustenite.

With such a method, it is possible to obtain the area ratio of each offerrite, bainite, martensite, residual austenite, and pearlite.

In the hot-rolled steel sheet according to the present embodiment, it isnecessary to control the structure observed using the optical microscopeto be within the above-described range, and to control the ratio of thegrains having an intragranular orientation difference in a range of 5°to 14°, obtained using an EBSD method (electron beam back scatteringdiffraction pattern analysis method) frequently used for the crystalorientation analysis. Specifically, in a case where a boundary havingthe orientation difference of equal to or higher than 15° is defined asa grain boundary, and an area which is surrounded by the grain boundaryand has an equivalent circle diameter of equal to or greater than 0.3 μmis defined as a grain, the ratio of the grains having an intragranularorientation difference in a range of 5° to 14° is set to be in a rangeof 10% to 60% by area ratio, with respect to the entire grains.

The grains having such intragranular orientation difference areeffective to obtain the steel sheet which has the strength and theworkability in the excellent balance, and thus when the ratio iscontrolled, it is possible to greatly improve the stretch flangeabilitywhile maintaining a desired steel sheet strength. When the ratio of thegrains having an intragranular orientation difference in a range of 5°to 14° is less than 10% by area ratio, the stretch flangeability isdeteriorated. In addition, when the ratio of the grains having anintragranular orientation difference in a range of 5° to 14° is greaterthan 60% by area ratio, the ductility is deteriorated.

Here, it is considered that an intragranular orientation difference isrelated to a dislocation density contained in the grains. Typically, theincrease in the intragranular dislocation density causes the workabilityto be deteriorated while bringing about the improvement of the strength.However, in the grain in which the intragranular orientation differenceis controlled to be in a range of 5° to 14°, it is possible to improvethe strength without deteriorating the workability. For this reason, inthe hot-rolled steel sheet according to the present embodiment, theratio of the grains having an intragranular orientation difference in arange of 5° to 14° is controlled to be in a range of 10% to 60%. Thegrains having an intragranular orientation difference of less lower 5°are excellent in the workability, but are hard to be highlystrengthened, and the grains having an intragranular orientationdifference of greater than 14° have different deformations therein, andthus do not contribute to the improvement of the stretch flangeability.

The ratio of the grains having an intragranular orientation differencein a range of 5° to 14° can be measured by the following method.

First, at a position of depth of ¼ (¼t portion) thickness t from surfaceof the steel sheet in a cross section vertical to a rolling direction,an area of 200 μm in the rolling direction, and 100 μm in the normaldirection to the rolled surface is subjected to EBSD analysis at ameasurement pitch of 0.2 μm so as to obtain crystal orientationinformation. Here, the EBSD analysis is performed using an apparatuswhich is configured to include a thermal field emission scanningelectron microscope (JSM-7001F, manufactured by JEOL) and an EBSDdetector (HIKARI detector manufactured by TSL), at an analysis speed ina range of 200 to 300 points per second. Then, with respect to theobtained crystal orientation information, an area having the orientationdifference of equal to or greater than 15° and an equivalent circlediameter of equal to or greater than 0.3 μm is defined as a grain, theaverage intragranular orientation difference of the grains iscalculated, and the ratio of the grains having an intragranularorientation difference in a range of 5° to 14° is obtained. The graindefined as described above and the average intragranular orientationdifference can be calculated using software “OIM Analysis (trademark)”attached to an EBSD analyzer.

The “intragranular orientation difference” of the present inventionmeans “Grain Orientation Spread (GOS)” which is an orientationdispersion in the grains, and the value thereof is obtained as anaverage value of reference crystal orientations and misorientations ofall of the measurement points within the same grain as disclosed in“Misorientation Analysis of Plastic Deformation of Stainless Steel byEBSD and X-Ray Diffraction Methods”, KIMURA Hidehiko, journal of theJapan Society of Mechanical Engineers (Series A) Vol. 71, No. 712, 2005,p. 1722 to 1728. In the present embodiment, the reference crystalorientation is an orientation obtained by averaging all of themeasurement points in the same grain, a value of GOS can be calculatedusing “OIM Analysis (trademark) Version 7.0.1” which is softwareattached to the EBSD analyzer.

FIG. 1 is an example of an EBSD analysis result of an area of 100 μm×100μm at ¼t portion in the cross section vertical to the rolling directionof the hot-rolled steel sheet according to the present embodiment. InFIG. 1, an area in which a boundary having the orientation difference ofequal to or greater than 15° is indicated as a grain boundary in a rangeof 5° to 14° is shown in gray. In the drawing, an area shown in blackindicates martensite.

In the present embodiment, the stretch flangeability is evaluated usingthe saddle type stretch flange test method in which the saddle-shapedformed product is used. Specifically, the saddle-shaped formed productsimulating the stretch flange shape formed of a linear portion and anarc portion as illustrated in FIG. 2 is pressed, and the stretchflangeability is evaluated using a limit forming height at this time. Inthe saddle type stretch flange test of the present embodiment, the limitforming height H (mm) when a clearance at the time of punching a cornerportion is set to 11% is measured using the saddle-type formed productin which a radius of curvature R of a corner is set to be in a range of50 to 60 mm, and an opening angle θ is set to 120°. Here, the clearanceindicates the ratio of a gap between a punching die and a punch, and thethickness of the test piece. Actually, the clearance is determined bycombination of a punching tool and the sheet thickness, and thus thevalue of 11% means a range of 10.5% to 11.5%. The existence of thecracks having a length of ⅓ of the sheet thickness are visually observedafter forming, and then a forming height of the limit in which thecracks are not present is determined as the limit forming height.

In a hole expansion test which is used as a test method to evaluate thestretch flange formability in the related art, breaking occurs withoutstrains are mostly distributed in the circumferential direction, andthus the strain and the gradient of stress in the vicinity of the brokenportion during hole expansion test are different from that in the caseof actually forming the stretch flange. In addition, in the holeexpansion test, the evaluation reflecting the original stretch flangeforming is not performed since, the evaluation is performed when therupture of the thickness penetration occurred. On the other hand, in thesaddle type stretch flange test used in the present embodiment, it ispossible to evaluate the stretch flangeability in consideration of thestrain distribution, and thus the evaluation reflecting the originalstretch flange forming can be performed.

In the hot-rolled steel sheet according to the present embodiment, thearea ratio of each of the structures of the ferrite and bainite whichare observed using the optical microscope is not directly related to theratio of the grains having an intragranular orientation difference in arange of 5° to 14°. In other words, for example, even if there arehot-rolled steel sheets in which the area ratio of ferrite and bainiteare the same each other, the ratio of the grains having an intragranularorientation difference in a range of 5° to 14° are not necessarily thesame. Accordingly, it is not possible to obtain the propertiescorresponding to the hot-rolled steel sheet according to the presentembodiment only by controlling the ferrite area ratio, the bainite arearatio, and the martensite area ratio. Details for this will be describedin Examples below.

The hot-rolled steel sheet according to the present embodiment can beobtained using a manufacturing method including a hot rolling processand a cooling process as follows.

<Hot Rolling Process>

In the hot rolling process, the hot-rolled steel sheet is obtained byheating and hot rolling a slab having the above-described chemicalcomposition. The slab heating temperature is preferably in a range of1050° C. to 1260° C. When the slab heating temperature is lower than1050° C., it is difficult to secure the hot rolling finishingtemperature, which is not preferable. On the other hand, when the slabheating temperature is equal to or higher than 1260° C., the yield isdecreased due to the scale off, and thus the heating temperature ispreferably equal to or lower than 1260° C.

In a case where the ratio of the grains having an intragranularorientation difference in a range of 5° to 14° is set to be in a rangeof 10% to 60% by area ratio, in the hot rolling performed on the heatedslab, it is important to set cumulative strains in a latter three stages(last three passes) of finish rolling to be greater than 0.6 to 0.7, andthen perform cooling described below. The reason for this is that sincethe grain having an intragranular orientation difference in a range of5° to 14° is generated by being transformed at a relatively lowtemperature in a para-equilibrium state, it is possible to control thegeneration of grain having an intragranular orientation difference in arange of 5° to 14° by limiting the dislocation density of austenitebefore the transformation to be in a certain range and limiting thecooling rate after transformation to be in a certain range. In otherwords, when the cumulative strain at the latter three stages in thefinish rolling, and the subsequent cooling are controlled, the grainnucleation frequency of the grain having an intragranular orientationdifference in a range of 5° to 14°, and the subsequent growth rate canbe controlled, and thus it is possible to control the area ratio whichis obtained as a result. More specifically, the dislocation density ofthe austenite introduced through the finish rolling is mainly related tothe grain nucleation frequency, and the cooling rate after rolling ismainly related to the growth rate.

When the cumulative strain at the latter three stages in the finishrolling is equal to or less than 0.6, the ratio of the grains having anintragranular orientation difference in a range of 5° to 14° is lessthan 10%, which is not preferable. Further, when the cumulative strainat the latter three stages in the finish rolling is greater than 0.7,the recrystallization of the austenite occurs during the hot rolling,the accumulated dislocation density at the time of the transformation isdecreased, and thus the ratio of the grains having an intragranularorientation difference in a range of 5° to 14° is less than 10%, whichis not preferable.

The cumulative strain (εeff.) at the latter three stages in the finishrolling in the present embodiment can be obtained from the followingEquation (1).

εeff.=Σεi(t,T)  (1)

Here,

εi(t,T)=εi0/exp{(t/tR)^(2/3)},

tR=t0×exp(Q/RT),

t0=8.46×10⁻⁶,

Q=183200 J, and

R=8.314 J/K·mol,

εi0 represents a logarithmic strain at the time of rolling reduction, trepresents a cumulative time immediately before the cooling in the pass,and T represents a rolling temperature in the pass.

The rolling finishing temperature of the hot rolling is preferably in arange of Ar3° C. to Ar3+60° C. When the rolling finishing temperature ishigher than Ar3+60° C., the grain size of the hot-rolled sheet becomeslarger, thus the workability is deteriorated, and the ratio of thegrains having an intragranular orientation difference in a range of 5°to 14° is decreased, which is not preferable. In addition, when therolling finishing temperature is lower than Ar3, the hot rolling isperformed in the two phase region, thus the ferrite phase is deformed,the ductility and the hole expansibility of the hot-rolled steel sheetare deteriorated, and the ratio of the grains having an intragranularorientation difference in a range of 5° to 140 is decreased, which isnot preferable.

Further, the hot rolling includes rough rolling and finish rolling, andthe finish rolling is preferably performed using a tandem mill withwhich a plurality of mills are linearly arranged and continuouslyrolling in one direction so as to obtain a desired thickness. Inaddition, in a case where the finish rolling is performed using a tandemmill, it is preferable that cooling (cooling between stands) isperformed between the mills such that the maximum temperature of thesteel sheet during the finish rolling is controlled to be in a range ofAr3+60° C. to Ar3+150° C. When the maximum temperature of the steelsheet during the finish rolling is higher than Ar3+150° C., the grainsize becomes excessively large and thus the toughness may bedeteriorated and the ratio of the grains having an intragranularorientation difference in a range of 5° to 14° may be decreased. On theother hand, when the maximum temperature of the steel sheet during thefinish rolling is lower than Ar3+60° C. there is a concern in that therolling finishing temperature of the finish rolling cannot be secured.

When the hot rolling is performed under the above-described conditions,the range of the dislocation density of austenite before thetransformation can be limited, and as a result, it is possible to obtaina desired ratio of the grains having an intragranular orientationdifference in a range of 5° to 14°.

Ar3 can be calculated by the following Expression (2) in considerationof the influence on the transformation point by rolling reduction.

Ar3=970−325×[C]+33×[Si]+287×[P]+40×[Al]−92×([Mn]+[Mo]+[Cu])−46×([Cr]+[Ni])  (2)

Here, [C], [Si], [P], [Al], [Mn], [Mo], [Cu], [Cr], and [Ni] eachrepresent, by mass %, the amount of each of C, Si, P, Al, Mn, Mo, Cu,Cr, and Ni. The elements which are not contained are calculated as 0%.

<Cooling Process>

In the hot-rolled steel sheet which was subjected to the hot rollingcontrolled as described above is cooled. In the cooling process, thehot-rolled steel sheet after completing the hot rolling is cooled (firstcooling) down to a temperature range in a range of 650° C. to 750° C. ata cooling rate of equal to or greater than 10° C./s, and the temperatureis kept for 3 to 10 seconds in the temperature range, and thereafter,the hot-rolled steel sheet is cooled (second cooling) down to thetemperature of equal to or lower than 100° C. at a cooling rate of equalto or greater than 30° C./s.

When the cooling rate in the first cooling is lower than 10° C./s, theratio of the grains having an intragranular orientation difference in arange of 5° to 14° is less than 10%, which is not preferable. Inaddition, when a cooling stopping temperature in the first cooling islower than 650° C., the ratio of the grains having an intragranularorientation difference in a range of 5° to 14° is less than 10%, whichis not preferable.

On the other hand, when the cooling stopping temperature in the firstcooling is higher than 750° C., the martensite fraction is excessivelylow, the strength is decreased, and the ratio of the grains having anintragranular orientation difference in a range of 5° to 14° is greaterthan 60%, which is not preferable. When the retention time is shorterthan three seconds at a temperature range of 650° C. to 750° C., themartensite fraction is excessively high, the ductility is deteriorated,and the ratio of the grains having an intragranular orientationdifference in a range of 5° to 14° is less than 10%, which is notpreferable. When the retention time at a temperature range of 650° C. to750° C. is longer than 10 seconds, the martensite fraction is decreased,the strength is deteriorated, and the ratio of the grains having anintragranular orientation difference in a range of 5° to 14° is lessthan 10%, which is not preferable. In addition, when the cooling rate ofthe second cooling is lower than 30° C./s, the martensite fraction isdecreased, the strength is deteriorated, and the ratio of the grainshaving an intragranular orientation difference in a range of 5° to 14°is greater than 60%, which is not preferable. When the cooling stoppingtemperature of the second cooling is higher than 100° C., the ratio ofthe grains having an intragranular orientation difference in a range of5° to 14° is greater than 60%, which is not preferable.

Although the upper limit of the cooling rate in the first cooling andthe second cooling is not necessarily limited, the cooling rate may beset to be equal to or lower than 200° C./s in consideration of theequipment capacity of the cooling facility.

According to the above-described manufacturing method, it is possible toobtain a structure which has, by area ratio, ferrite and bainite in arange of 75% to 95% in total, and martensite in a range of 5% to 20%, inwhich a boundary having an orientation difference of equal to or greaterthan 15° is set as a grain boundary, and in a case where an area whichis surrounded by the grain boundary, and has an equivalent circlediameter of equal to or greater than 0.3 μm is defined as a grain, theratio of the grains having an intragranular orientation difference in arange of 5° to 14° is, by area ratio, in a range of 10% to 60%.

In the aforementioned manufacturing method, it is important thatprocessed dislocations are introduced into the austenite by controllingthe hot rolling conditions, and then the processed dislocationsintroduced by controlling the cooling conditions appropriately remain.That is, the hot rolling conditions and the cooling conditions each havean influence, it is important to control these conditions at the sametime. A known method may be used for conditions other than theabove-described ones, and there is no particular limitation.

Examples

Hereinafter, the present invention will be described more specificallywith reference to examples of the hot-rolled steel sheet of the presentinvention. However, the present invention is not limited to Exampledescribed below, and can be implemented by being properly modified theextent that it can satisfy the object before and after description,which are all included in the technical range of the present invention.

First, the steel having the chemical composition indicated in thefollowing Table 1 was melted, and continuous cast so as to produce aslab. Then, the slab was heated at a temperature indicated in Table 2,and was subjected to rough rolling. After the rough rolling, the finishrolling was performed under the conditions indicated in Table 2 so as toobtain a hot-rolled steel sheet having the sheet thickness in a range of2.2 to 3.4 mm. Ar3 (° C.) indicated in Table 2 was obtained from thechemical composition indicated in Table 1 using the following Expression(2).

Ar3=970−325×[C]+33×[Si]+287×[P]+40×[Al]−92×([Mn]+[Mo]+[Cu])−46×([Cr]+[Ni])  (2)

Here, [C], [Si], [P], [Al], [Mn], [Mo], [Cu], [Cr], and [Ni] eachrepresent, by mass %, the amount of each of C, Si, P, Al, Mn, Mo, Cu,Cr, and Ni, by mass %, and in a case of not containing the elements, 0is substituted.

In addition, in Table 2, the cumulative strains at the latter threestages of the finish rolling were the value obtained by the followingExpression (1).

εeff.=Σεi(t,T)  (1)

Here,

εi(t,T)=εi0/exp{(ti/tR)^(2/3)},

tR=t0·exp(Q/RT),

t0=8.46×10⁻⁶,

Q=183200 J, and

R=8.314 J/K·mol,

εi0 represents a logarithmic strain at the time of rolling reduction, trepresents a cumulative time immediately before the cooling in the pass,and T represents a rolling temperature in the pass.

The blank column in Table 1 means that the analysis value was less thanthe detection limit.

TABLE 1 Steel Chemical compositions (mass %, remainder: Fe andimpurities) Ar3 No. C Si Mn P S Al N B Cr Mo Cu Ni Mg REM Ca Zr (° C.) A0.06 0.90 1.90 0.018 0.005 0.35 0.0018 825 B 0.06 1.20 1.20 0.030 0.0020.030 0.0021 0.10 885 C 0.07 0.50 1.80 0.010 0.003 0.25 0.0020 0.06 0.03804 D 0.06 1.00 1.10 0.030 0.004 0.25 0.0031 901 E 0.09 0.90 0.90 0.0200.003 0.030 0.0028 0.0005 895 F 0.09 0.30 1.00 0.015 0.004 0.040 0.00250.15 858 G 0.08 0.80 1.60 0.009 0.004 0.30 0.0032 838 H 0.12 1.00 1.500.030 0.003 0.040 0.0038 0.0005 836 I 0.10 0.40 0.80 0.012 0.003 0.0300.0020 882 J 0.14 0.50 1.10 0.006 0.002 0.030 0.0026 0.13 0.0003 831 K0.10 0.70 0.80 0.013 0.003 0.020 0.0031 0.20 882 L 0.09 0.90 0.90 0.0150.003 0.040 0.0028 894 M 0.08 1.20 1.30 0.013 0.004 0.030 0.0018 0.0005869 N 0.09 0.80 1.50 0.012 0.003 0.050 0.0020 0.0005 835 O 0.06 1.201.30 0.010 0.005 0.030 0.0042 875 P 0.07 0.90 1.10 0.011 0.004 0.1000.0035 0.30 869 Q 0.08 1.00 0.90 0.015 0.005 0.030 0.0040 0.10 0.05 888a 0.23 0.50 1.30 0.010 0.003 0.030 0.0018 796 b 0.05 2.30 0.70 0.0150.003 0.030 0.0022 971 c 0.16 0.80 3.50 0.013 0.004 0.040 0.0025 0.15621 d 0.01 0.50 0.90 0.016 0.003 0.030 0.0043 0.0020 906 e 0.10 1.001.20 0.015 0.005 0.020 0.0038 0.0200 865 f 0.09 0.90 0.20 0.018 0.0030.030 0.0023 0.0006 958 Underlines represent being outside of the rangedefined in the present invention

TABLE 2 Maximum Difference between Cumulative temperature of steelHeating Rolling finishing rolling finishing strains at later sheetduring finish Test Steel Ar3 temperature temperature temperature and Ar3three stages after rolling No. No. (° C.) (° C.) (° C.) (° C.) finishrolling (° C.) 1 A 825 1150 882 57 0.678 957 2 B 885 1150 923 38 0.6231012 3 C 804 1150 845 41 0.665 947 4 D 901 1200 912 11 0.618 1030 5 E895 1150 926 31 0.623 1030 6 F 858 1100 900 42 0.666 1000 7 G 838 1170878 40 0.682 975 8 H 836 1150 890 54 0.643 982 9 I 882 1120 910 28 0.6351020 10 J 831 1140 889 58 0.614 970 11 K 882 1150 923 41 0.623 1010 12 L394 1150 943 49 0.65 1020 13 M 869 1160 919 50 0.679 1000 14 N 835 1150894 59 0.647 981 15 O 875 1090 930 55 0.653 1018 16 P 869 1150 918 490.629 1002 17 Q 888 1180 923 35 0.691 1030 18 a 796 1200 827 31 0.624927 19 b 971 1150 955 −16 0.610 1050 20 c 621 1150 820 199 0.654 950 21d 906 1100 936 30 0.672 1002 22 e 865 1180 895 30 0.601 1000 23 f 9581150 988 30 0.615 1085 24 B 885 1000 875 −10 0.623 1002 25 B 885 1150840 −45 0.658 987 26 B 885 1160 950 65 0.653 1001 27 B 885 1090 917 320.473 1013 28 B 885 1100 920 35 0.821 1005 29 B 885 1240 925 40 0.6271120 30 B 885 1150 900 15 0.674 1034 31 D 901 1150 920 19 0.662 1017 32D 901 1160 913 12 0.632 1010 33 H 836 1120 889 53 0.654 982 34 H 8361130 890 54 0.682 980 35 O 875 1150 895 20 0.612 1010 36 O 875 1100 90732 0.692 1000 Cooling stopping Retention time at a Cooling stoppingCooling rate in temperature in temperature range of Cooling rate intemperature in Test first cooling first cooling 650° C. to 750° C.second cooling second cooling No. (° C./s) (° C.) (seconds) (° C./s) (°C.)  1 15 670 4 35 90  2 20 680 5 40 80  3 38 700 6 46 70  4 42 720 5 5080  5 18 730 6 35 50  6 25 700 7 62 30  7 41 660 5 40 30  8 40 680 4 4530  9 35 690 3 60 40 10 40 700 8 33 30 11 50 710 7 38 60 12 30 720 5 4040 13 28 730 9 36 80 14 36 740 7 41 50 15 19 690 5 43 70 16 26 680 4 5660 17 30 670 6 37 80 18 25 690 9 37 80 19 15 720 5 43 50 20 25 720 6 3850 21 30 680 7 36 80 22 25 700 7 40 70 23 40 680 5 39 40 24 18 670 4 5060 25 32 670 6 35 70 26 19 700 6 43 80 27 22 690 7 47 70 28 21 710 4 3370 29 18 740 9 41 80 30 3 710 7 49 80 31 32 820 5 46 70 32 25 600 9 3840 33 19 690 1 34 50 34 32 670 15 37 60 35 35 700 7 2 80 36 27 680 6 38500

With respect to the obtained hot-rolled steel sheet, each structurefraction (the area ratio), and the ratio of the grains having anintragranular orientation difference in a range of 5° to 14° wereobtained. The structure fraction (the area ratio) was obtained using thefollowing method. First, a sample collected from the hot-rolled steelsheet was etched using nital. After etching, a structure photographobtained at a ¼ thickness position in a visual field of 300 μm×300 μmusing an optical microscope was subjected to image analysis, and therebythe area ratio of ferrite and pearlite, and the total area ratio bainiteand martensite were obtained. Then, With a sample etched by LePerasolution, the structure photograph obtained at a ¼ thickness position inthe visual field of 300 μm×300 μm using the optical microscope wassubjected to the image analysis, and thereby the total area ratio ofresidual austenite and martensite was calculated.

Further, with a sample obtained by grinding the surface to a depth of ¼thickness in the normal direction to the rolled surface, the volumefraction of the residual austenite was obtained through X-raydiffraction measurement. The volume fraction of the residual austenitewas equivalent to the area ratio, and thus was set as the area ratio ofthe residual austenite.

With such a method, the area ratio of each of ferrite, bainite,martensite, residual austenite, and pearlite was obtained.

Further, the ratio of the grains having an intragranular orientationdifference in a range of 5° to 14° was measured using the followingmethod. First, at a position of depth of ¼ (¼t portion) thickness t fromsurface of the steel sheet in a cross section vertical to a rollingdirection, an area of 200 μm in the rolling direction, and 100 μm in thenormal direction to the rolled surface was subjected to EBSD analysis ata measurement pitch of 0.2 μm so as to obtain crystal orientationinformation. Here, the EBSD analysis was performed using an apparatuswhich is configured to include a thermal field emission scanningelectron microscope (JSM-7001F, manufactured by JEOL) and an EBSDdetector (HIKARI detector manufactured by TSL), at an analysis speed ina range of 200 to 300 points per second. Then, with respect to theobtained crystal orientation information, an area having the orientationdifference of equal to or greater than 15° and an equivalent circlediameter of equal to or greater than 0.3 μm was defined as a grain, theaverage intragranular orientation difference of the grains wascalculated, and the ratio of the grains having an intragranularorientation difference in a range of 5° to 14° was obtained. The graindefined as described above and the average intragranular orientationdifference can be calculated using software “OIM Analysis (trademark)”attached to an EBSD analyzer.

Next, the yield strength and the tensile strength were obtained in thetensile test, and the limit forming height was obtained by the saddletype stretch flange test. In addition, a product of tensile strength(MPa) and limit forming height (mm) was evaluated as an index of thestretch flangeability, and in a case where the product thereof is equalto or greater than 19500 mm·MPa, it was determined that the steel sheetwas excellent in the stretch flangeability.

The tensile test was performed based on JIS Z 2241 using tensile testpieces No. 5 of JIS which were collected in the longitudinal directionwhich is orthogonal to the rolling direction.

Further, the saddle type stretch flange test was conducted by setting aclearance at the time of punching a corner portion to 11% using asaddle-type formed product in which a radius of curvature R of a cornerwas set to 60 mm, and an opening angle θ was set to 120°. In addition,the existence of the cracks having a length of ⅓ of the sheet thicknesswere visually observed after forming, and then a forming height of thelimit in which the cracks were not present was determined as the limitforming height.

The results are indicated in Table 3.

TABLE 3 Ferrite + Ratio of the grains having Ferrite Bainite bainiteMartensite intragranular orientation area area area area differenceYield Tensile Index of stretch Test ratio ratio ratio ratio in a rangeof 5° 14° strength strength flange No. (%) (%) (%) (%) (%) (MPa) (MPa)(mm · MPa) Remarks 1 45 42 87 13 56 525 821 21346 Example of Presentinvention 2 87 6 93  7 31 372 602 21672 Example of Present invention 355 33 88 12 42 541 873 20079 Example of Present invention 4 90 3 93  733 412 610 21350 Example of Present invention 5 79 16 95  5 41 450 65220864 Example of Present invention 6 56 34 90 10 46 512 800 21600Example of Present invention 7 42 45 87 13 58 543 817 21242 Example ofPresent invention 8 48 42 90 10 58 551 810 22680 Example of Presentinvention 9 67 24 91  9 47 500 787 21249 Example of Present invention 1038 51 89 11 21 531 850 21250 Example of Present invention 11 42 51 93  728 569 830 20750 Example of Present invention 12 86 8 94  6 59 426 64022400 Example of Present invention 13 80 13 93  7 57 411 632 20856Example of Present invention 14 52 38 90 10 51 510 810 20250 Example ofPresent invention 15 88 4 92  8 46 399 609 21924 Example of Presentinvention 16 69 18 87 13 32 393 645 20640 Example of Present invention17 83 5 88 12 57 372 600 20400 Example of Present invention 18 0 0  0100   0 918 997 3988 Comparative Example 19 95 5 96  0  7 345 459 16065Comparative Example 20 3 42 45 65  4 820 1120 11200 Comparative Example21 88 10 98  2  5 276 460 18860 Comparative Example 22 20 58 78 18  3689 899 13485 Comparative Example 23 90 6 96  4 15 292 463 17594Comparative Example 24 88 6 93  7  2 380 585 18720 Comparative Example25 82 13 95  5  1 418 592 17760 Comparative Example 26 56 22 78 22  2526 813 16432 Comparative Example 27 84 3 87  8  1 405 610 15250Comparative Example 28 79 4 83 17  4 378 593 17790 Comparative Example29 78 14 92  8  2 403 605 17545 Comparative Example 30 67 19 86 14  3410 613 18390 Comparative Example 31 79 18 97  3 78 408 575 18975Comparative Example 32 72 9 81 19  6 411 623 18690 Comparative Example33 46 26 72 28  5 570 812 18676 Comparative Example 34 90 7 97  3  7 510750 18750 Comparative Example 35 88 4 92  2 62 432 549 18666 ComparativeExample 36 89 11 100   0 73 495 582 11640 Comparative Example

As apparent from the results of Table 3, in a case where the slabincluding the chemical composition specified in the present inventionwas hot-rolled under the preferable conditions (Test Nos. 1 to 17), itwas possible to obtain the high-strength hot-rolled steel sheet in whichthe strength is equal to or greater than 590 MPa, and an index of thestretch flangeability is equal to or greater than 19500 mm·MPa.

On the other hand, regarding Manufacture Nos. 18 to 23, the chemicalcomposition was outside the range of the present invention, and thus anyone or both of the structure observed using the optical microscope andthe ratio of the grains having an intragranular orientation differencein a range of 5° to 14° did not satisfy the range of the presentinvention. As a result, the stretch flangeability did not satisfy thetarget value. In addition, in some examples, the tensile strength isalso decreased.

In addition, Nos. 24 to 36 are examples in which the manufacturingmethod was outside the preferable range, and thus any one or both of thestructure observed using the optical microscope and the ratio of thegrains having an intragranular orientation difference in a range of 5°to 14° did not satisfy the range of the present invention. In theseexamples, the stretch flangeability did not satisfy the target value. Inaddition, in some examples, the tensile strength was also decreased.

INDUSTRIAL APPLICABILITY

According to the present invention, it is possible to provide anhigh-strength hot-rolled steel sheet which is excellent in the stretchflangeability and can be applied to a member which requires highstrength and the strict stretch flangeability. The steel sheetcontributes to improving fuel economy of vehicles, and thus has highindustrial applicability.

1. A hot-rolled steel sheet comprising, as a chemical composition, bymass %, C: 0.04% to 0.18%, Si: 0.10% to 1.70%, Mn: 0.50% to 3.00%, Al:0.010% to 1.00%, B: 0% to 0.005%, Cr: 0% to 1.0%, Mo: 0% to 1.0%, Cu: 0%to 2.0%, Ni: 0% to 2.0%, Mg: 0% to 0.05%, REM: 0% to 0.05%, Ca: 0% to0.05%, Zr: 0% to 0.05%, P: limited to equal to or less than 0.050%, S:limited to equal to or less than 0.010%, and N: limited to equal to orless than 0.0060%, with the remainder including Fe and impurities;wherein a structure includes, by area ratio, a ferrite and a bainite ina range of 75% to 95% in total, and a martensite in a range of 5% to20%, and wherein in the structure, in a case where a boundary having anorientation difference of equal to or greater than 15° is defined as agrain boundary, and an area which is surrounded by the grain boundaryand has an equivalent circle diameter of equal to or greater than 0.3 μmis defined as a grain, the ratio of the grains having an intragranularorientation difference in a range of 5° to 14° is, by area ratio, in arange of 10% to 60%.
 2. The hot-rolled steel sheet according to claim 1,wherein a tensile strength is equal to or greater than 590 MPa, and aproduct of the tensile strength and a limit forming height in a saddletype stretch flange test is equal to or greater than 19500 mm·MPa. 3.The hot-rolled steel sheet according to claim 1 or 2, wherein thechemical composition contains, by mass %, one or more selected from thegroup consisting of: B: 0.0001% to 0.005%, Cr: 0.01% to 1.0%, Mo: 0.01%to 1.0%, Cu: 0.01% to 2.0%, and Ni: 0.01% to 2.0%.
 4. The hot-rolledsteel sheet according to claim 1 or 2, wherein the chemical compositioncontains, by mass %, one or more selected from the group consisting of:Mg: 0.0001% to 0.05%, REM: 0.0001% to 0.05%, Ca: 0.0001% to 0.05%, andZr: 0.0001% to 0.05%.
 5. The hot-rolled steel sheet according to claim3, wherein the chemical composition contains, by mass %, one or moreselected from the group consisting of: Mg: 0.0001% to 0.05%, REM:0.0001% to 0.05%, Ca: 0.0001% to 0.05%, and Zr: 0.0001% to 0.05%.